・$\frac{\pi^2}{6} = \displaystyle \prod_{p;prime} \frac{1}{1-\frac{1}{p^2}} = \left ( \frac{1}{1-\frac{1}{2^2}} \right ) \left ( \frac{1}{1-\frac{1}{3^2}} \right ) \left ( \frac{1}{1-\frac{1}{5^2}} \right ) \left ( \frac{1}{1-\frac{1}{7^2}} \right ) \left ( \frac{1}{1-\frac{1}{11^2}} \right ) \cdots$
$\frac{\pi^2}{6} =$